There are a hack or two for this purpose, that may do the trick. I run into these things all the time so I thought i would post them.
The first allows you to search for a user-name in your member-list, it can be found here.
Member-list Find User
And here is a screen-shot also.
And the second enables people to search for users that have their name starting with a specific letter. You can also choose "others" (meaning non-alpha characters), and "all" (the regular list). Here is a link to that one.
Member-list Sortable by Letter
Crissy